1. Technical Field
The present invention relates to a gas cell and a coating method of the gas cell.
2. Related Art
Light pumping magnetic sensors have been used as biomagnetic measurement equipment for detecting a magnetic field generated from a heart of a living body or the like. Patent Document 1 (JP-A-2009-236599) discloses a gas cell, a magnetic sensor using pump light and probe light. In the magnetic sensor, atoms enclosed in the gas cell are excited by the pump light and spin polarization is caused. The polarization plane of the probe light transmitted through the gas cell rotates in response to a magnetic field, and thus, the magnetic field is measured using the rotation angle of the polarization plane of the probe light. Further, as a method of coating inner wall surfaces of the gas cell, for example, Non-patent Document 1 (M. A. Bouchiat and J. Brossel, “Relaxation of Optically Pumped Rb Atoms on Paraffin-Coated Walls”, Physical Review Vol. 147, No. 1, pp. 41-54 (1965) discloses an anti-relaxation coated cell using paraffin.
In the technology disclosed in Non-patent Document 1, the number of collisions of atoms with cell inner walls within a time period in which the spin polarization of the atoms disappears is used as an indicator representing anti-relaxation performance of the coating. Its value is about 10,000 and the paraffin coating has a sufficient effect for improvement of magnetic field sensitivity. However, there has been a problem that the anti-relaxation characteristics are deteriorated when the cell is heated to 50° C. or higher.
The paraffin molecule has a structure with a relatively long normal chain, and thus, strong attractive forces act in the entire molecules even when the van der Waals's forces between the atoms are weak. However, the inner wall surfaces of the gas cell (formed principally using borosilicate glass or quartz) are covered by polar groups represented by silanol groups, and thus, it is estimated that, while the attractive forces of the van der Waals's forces or the like acts on the paraffin molecules, they are easily separated by the hydrophobic effect and the adsorbed state is easily changed by heating.
On the other hand, not only the physical adsorption of paraffin but also silane-series materials forming a coating layer by bonding by chemical reaction with the inner walls of the gas cell have been proposed. Specifically, OTS (octadecyltrichlorosilane, C18H37C13Si) or the like is used. The material has good temperature characteristics and its heat resistance up to about 150° C. is confirmed. However, the number of collisions until the spin polarization disappears is about 2,100 at most and lower than that of paraffin.